Here's an interesting recipe. Take a spoonful of graphite powder and stir it into a glass of water. Leave for 24 hours at room temperature and then filter the powder. Finally, bake overnight at 100 degrees C and allow to cool.

And voila! A material that superconducts at over 300 kelvin--room temperature. At least that's the claim today from Pablo Esquinazi and buddies at the University of Leipzig in Germany. If that sounds too good to be true, it's worth taking a look at the claim in more detail since there are more than a few caveats. First, this is not a conventional bulk material.

The claim from Germany is that the superconductivity occurs at the interface between grains of graphite after they have dried out.

So that's a surface effect which involves only a tiny fraction of the total mass of carbon in the powder--just 0.0001 per cent of the mass, according to Esquinazi and co.

What's more the effect is clearly fragile. Esquinazi and co say the superconductivity disappears if the treated powder is pressed into pellets. So whatever allows the superconductivity to occur at the grain interfaces is destroyed when the grains are pressed together.

Finally, the experimental evidence is tantalising rather than definitive. In ordinary circumstances, claims for superconductivity require three different lines of evidence. First, there is zero resistance. Second there is the Meissner effect in which the sample reflects an external magnetic field. And finally there must be evidence of a superconducting phase transition, such as a sudden change in the material’s magnetic properties when superconductivity occurs. It's this final effect that Esquinazi and co discuss in their paper. They say that the material's magnetic properties such as its magnetic moment change in a way that is consistent with the presence of superconducting vortices.